The passage of adult years is associated with a gradual loss of body cell mass and even greater erosion of skeletal muscle mass. Since the metabolic function of the whole organism rests with adequate protein and energy intake to sustain or maintain these pools for organ function, a definition of these needs is paramount for convalescence. To this end, these requirements have been addressed in various age populations including the elderly using nitrogen balance methods. There also appears to be some extrapolation of data from younger populations in defining the overall nutrient need of the geriatric patient; however, no data are available on the effect of injury and illness on these requirements in older patients. This question becomes acute since we and others have shown that the metabolic response to injury (non-geriatric) is exemplified in an increased resting metabolic expenditure and increased loss of nitrogen in the urine after injury and illness. Associated with these increases is an increase in protein synthesis and breakdown; however, the breakdown rate predominates. The limited studies of protein kinetics in the normal geriatric patient and the complete absence of such data following injury suggest that such studies be considered. We have developed a metabolic facility for measuring nitrogen and calorie balances as well as protein and other substrate turnovers in a clinical environment. An evaluation of a group of control geriatric patients will provide valuable information as to the requirements of protein and energy in such a population as well as focus on the needs following insult. These data will help resolve the optimum fuel mixture to meet requirements in the geriatric patient following injury and illness.